Approximately half of the current young generation in the Western world will at some point in their lives be diagnosed as having cancer and this frequency is slowly increasing. More than half of these patients are likely to receive radiation therapy, due in particular to the increasing use and efficacy of intensity modulated radiation therapy (IMRT). Both the rising costs for cancer care and the adverse side-effects in normal tissues call for more locally effective treatment procedures such as radiation therapy, which is undoubtedly more cost-effective and generally more curative than both surgery or chemotherapy, and is being recommended more and more extensively. Extensive and mutilating surgery is now often replaced by minimally invasive surgery and will in the future be superseded by highly precise intensity modulated photon and light ion radiation therapy.
Improvements in tumor diagnostics, including four-dimensional computed tomography (4D-CT), magnetic resonance imaging (MRI) and the combination of positron emission tomography (PET) with CT (4D-PET-CT), both enhance our knowledge about tumor spread in relation to normal tissues and allow more precise delivery of radiation, thereby optimizing the treatment at a reasonable cost.
Furthermore, intensity modulated radiation therapy is rapidly becoming the treatment of choice for most tumors with respect to minimizing damage to the normal tissues and maximizing tumor control. Today, intensity modulated beams are most commonly delivered using segmental multileaf collimation, although an increasing number of radiation therapy departments are employing dynamic multileaf collimation. The irradiation time using dynamic multileaf collimation depends strongly on the nature of the desired dose distribution, and it is difficult to reduce this time to less than the sum of the irradiation times for all individual peaks heights using dynamic leaf collimation. Therefore, the intensity modulation will considerably increase the total treatment time.
Document [1] discloses a collimator arrangement consisting of a primary collimator, a multileaf collimator and two pairs of independently adjustable block diaphragm leaves at right angles to each other. By using rectilinear displacement of the multileaf collimator leaves and the block diaphragm leaves, a compact arrangement can be provided which fits into a standard collimator head. The diaphragm leaves can, in order to reduce the cost, have a respective inner portion made of tungsten while the remainder of the leaves is made of lead.